There has been known a developing unit including a developing housing and a toner cartridge freely detachably mounted on the developing housing. In one type of developing unit, toner is supplied through a toner outlet formed in the toner cartridge into the developing housing through a toner inlet formed therein. The toner cartridge has a shutter that selectively opens and closes the toner outlet, and the developing housing has another shutter that selectively opens and closes the toner inlet.
When the toner cartridge is mounted onto the developing housing, the shutter of the toner cartridge engages with an engaging member formed on the developing housing, and the shutter of the developing housing engages with an engaging member formed on the toner cartridge. In this condition, a user grips a part of the toner cartridge furthest from the toner outlet and then pivots the toner cartridge in a predetermined direction about a side nearest the toner outlet. This pivoting movement opens the both shutters at once. On the other hand, pivoting the toner cartridge in an opposite direction closes the both shutters at once, enabling the user to remove the toner cartridge from the developing housing.
That is, in the above-described developing unit, pivoting the toner cartridge for mounting the toner cartridge onto or removing the toner cartridge from the developing housing also opens or closes the both shutters.